> That isn't much different

There's a world of difference here. Ofcom is claiming to be able to shut down an American website for content generated in America, stored in America, and shown only to Americans. There are no UK citizens in this chain at all. This sets up Ofcom as having global censorship authority even over content seen elsewhere.

> Should they be prosecuted in that country for having drank in America?

In my opinion, no, but some countries are hardasses about this. If you want to do things that are illegal in certain places, you should not plan on traveling to those places. Usually, they will just refuse you entry but you kind of do put yourself at their mercy if you touch their soil. This is how the world works.